Why Is Sex Fun?: The Evolution Of Human Sexuality (Science Masters Series) by Jared M. Diamond.

Many of us pursue fitness because we want to remain attractive to partners and potential partners, and we stay healthy so we can continue to have sex with those partners. But why do people care so much about sex? This book, written by an evolutionary biologist, explains how all the weird quirks of human sexuality came to be: sex with no intention of procreation, invisible fertility, sex acts pursued in private–all common to us, but very different from most other species. Why Is Sex Fun? asks us to look at ourselves in a brand-new way, and richly rewards us for doing so.